function greeter(person:string) {
  return 'hello,' + person
}


const user = "Jane"
const sum = (a: number) => (b: number) => a + b
// 等价于
const sum2 = function (a:number){
  return function add(b:number){
    return a + b
  }
}

console.log(sum(1)(2))

const a:void = undefined //只能把undefined赋值给void

function v():void{ // 无返回值（即返回undefined）的funcition返回类型也是void
  console.log(123)
}

function s():string{ //一个返回string的function
  return ''
}

let undf:undefined = undefined

let nul:null = null

